Molecules of different gases have different electromagnetic radiation adsorption characteristics. A gas consisting of a given gas molecule therefore has a unique radiation adsorption spectrum. In one type of spectroscopic measurement of gases, the frequency of a radiation source is scanned over a defined frequency range, the resulting radiation is passed through a sample of a gas so that some of the radiation is absorbed by the gas, and the radiation which emerges from the gas sample is detected. The detected radiation may, for example, be indicative of such properties of the sample gas as gas concentration, gas composition, gas pressure and gas temperature.
Laser diodes are commonly used as a radiation source in gas spectroscopic devices to generate electromagnetic radiation of a single frequency and of a narrow linewidth. Laser radiation emitted from the exit aperture of a laser diode at the semiconductor chip surface, however, typically has a wide angle of divergence. As a consequence, the intensity of laser radiation incident on an object decreases rapidly as the object is placed farther and farther away from the laser diode. To reduce this decrease in intensity, it is desirable to reduce the divergence of the radiation emitted from a laser diode so that an adequate amount of laser radiation can be detected after the laser radiation has passed through the sample gas.
Many laser diode "collimator" configurations have been developed. One type of conventional laser diode "collimator" involves one or more lenses which receive the diverging radiation from the laser diode and collimate that radiation into a beam. The distance between the laser diode and the lenses must be controlled accurately or the beam will either focus or diverge. Due to tight mechanical tolerances required to produce a precisely collimated beam, conventional laser diode collimators typically have a lens focussing mechanism for adjusting the distance between the lens and the laser diode so that a desired degree of divergence or focus can be set and maintained. One such device is disclosed in FIG. 1 of U.S. Pat. No. 4,498,737 where precision adjustment of the laser diode to lens distance is accomplished through use of a threaded screw 4.
FIG. 1A (Prior Art) shows a second laser diode collimator of the prior art. A laser diode semiconductor chip 1A and lens 2A are contained in cylindrical supporting package 3A. The positioning of lens 2A in the X-Y plane relative to radiation emitted from the laser diode determines the direction beam 4A will point when it emerges from the lens. Lens 2A is, however, substantially fixed in the X-Y plane by the structure of the collimator package. The positioning of lens 2A in the Z dimension relative to the laser diode, determines whether beam 4A emerging from the lens is divergent, collimated, or focussed. The distance between lens 2A and laser diode 1A can be adjusted by rotating focus screw 5A. Once the lens is positioned in the Z dimension as desired, the lens is locked in place by locking screw 6A. When current is supplied to laser diode chip 1A on leads 7A and 8A, diverging laser radiation 10A is emitted from laser diode 1A. This radiation passes through transparent window 9A, and is redirected by lens 2A into laser beam 4A of a controlled divergence.
Due to their adjustability, however, such prior art laser diode collimators are susceptible to disalignment and decalibration problems. Physical shocks, physical vibrations, and forces resulting from temperature changes and pressure changes may cause the lens to move with respect to the laser diode. As a result, the relative divergence or focus as well as the pointing direction of beam 4A changes thereby resulting in spatial changes in the incident radiation on an object positioned at a given distance from the lens. Moreover, providing an adjustment and lens locking mechanism such as is shown in FIG. 1A adds complexity, size and significant expense to the collimator.
FIG. 1B (Prior Art) shows another type of collimator known in the prior art which sacrifices X-Y adjustability in order to reduce complexity. One such structure is offered by Diverse Optics of San Dimas, Calif. This type of collimator, however, does involve a mechanism for adjusting the distance between the laser diode 1B and the lens 2B so that the exit beam 4B can be precisely adjusted to produce a predefined divergence. Collimating lens 2B is mounted in plastic package 11B that is designed to mount tightly over a commonly available laser diode housing 12B. The plastic package is designed to slide with frictional resistance down cylindrical outer surface 13B of the laser diode housing 12B until the desired laser diode to lens distance in the Z dimension is achieved as evidenced by the character of the exit beam 4B. Upon observance of the proper exit beam divergence, adjustment is stopped and the tight fit of the plastic package in contact with outer surface 13B of the housing in area 14B insures that the lens 2B within the plastic package lib will maintain the desired laser diode to lens distance and therefore will continue to produce an exit beam of the desired divergence. Because the structure of FIG. 1B does not provide fore accurate adjustment of the lens in the X-Y plane, the direction in which the exit beam 4B points is variable and is governed by the accuracy of the mounting of lens 2B in plastic package 11B as well as the centering accuracy of laser diode chip 1B with respect to cylindrical diode housing 12B.